Non-executive director Stan Kroenke has strengthened his involvement with Arsenal by upping his stake in the club to 20%. Photograph: Garrett W. Ellwood/Getty Images

The American tycoon Stan Kroenke has increased his stake in Arsenal to just over 20% after buying 5,000 shares from one of his ­fellow directors, Danny Fiszman, for £42.5m.

Kroenke was able to increase his stake in the club after the Arsenal board gave its consent to the transaction, which was set at a price of £8,500 per share.

With a further 25% of the club's shares owned by the Russian metals magnate Alisher Usmanov, almost half of the club is now under foreign ownership. Fiszman, who still owns some 10,025 shares – around 16.1% of the total capital of Arsenal's parent holding company – insists he has no intention of selling any more of his stake. The shares owned by the former board member Lady Nina Bracewell-Smith, however, are thought to be available for purchase.

"I am pleased that Stan Kroenke has made a further substantial commitment to the club by acquiring approximately a third of my holding," Fiszman said.

"Stan's long-term commitment to sport in general and football in particular has been well documented. I am therefore delighted that he has shown this desire to deepen his ties with Arsenal. I will of course continue to work for Arsenal with the best interests of the club at heart and have no intention of selling any more of my shareholding."

Kroenke, who accepted an offer to become a non-executive director in September 2008, as ties with his Kroenke Sports Enterprises group were strengthened, believes the club is heading in the right direction.

"After having been invited to join the board last year I am delighted to be able to increase my shareholding in Arsenal," he said. "I will continue to work closely with my board colleagues to maintain the stable environment in which the club operates and to preserve the self-sustaining business model enjoyed by the club."

The Arsenal chairman, Peter Hill-Wood, said: "Stan Kroenke has proved to be a valuable member of the board and I am pleased that he has demonstrated further commitment to the club by adding to his shareholding. Danny Fiszman remains a driving influence on the board and is fully committed to the club's long-term future."

In February, Usmanov's Red and White Holdings group increased its stake in the club to 25%. If that was to reach 30%, under financial regulations a formal takeover approach would have to be launched.

Lady Bracewell-Smith is thought to be ready to listen to offers for some of her 15.9% holding, after leaving the club's board in acrimonious circumstances in December.